A sugar of molar mass 150 g/mol contains 40.0% carbon, 6.7% hydrogen and 53.3% oxygen by mass…
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
A sugar of molar mass contains carbon, hydrogen and oxygen by mass. Take C as , H as and O as . Find the empirical formula, then the molecular formula. If the sugar is an aldopentose with an unbranched chain, how many stereoisomers does it have?
Need a reminder? The key ideaShow
A carbohydrate is a polyhydroxy aldehyde or ketone. An aldose has stereocentres and a ketose has , where is the number of carbons in the chain. The number of stereoisomers is , and half of them belong to the D series. An empirical formula comes from the moles of each element in 100 g.
The worked solution
Have you tried it?
Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.
Plan. Moles of each element give the empirical formula. The molar mass gives the multiplier.
Step 1. Moles in 100 g. Carbon: . Hydrogen: . Oxygen: .
Step 2. Ratio. Dividing by 3.33 gives C : H : O = 1 : 2 : 1. The empirical formula is , with a molar mass of .
Step 3. Molecular formula. . The formula is , so there are 5 carbon atoms.
Step 4. Stereoisomers. An aldopentose has 3 stereocentres, so stereoisomers.
Answer: , with 8 stereoisomers if it is an aldopentose.
Check. The hydrogen is twice the oxygen in the ratio, as in every carbohydrate of this kind: .
Discussion (0)
Log in to join the discussion.
Loading…